It is well known that an aircraft body in flight creates a body wake, which is the disturbance in downstream flow resulting from the air flowing over the aircraft body. A proportion of the body wake results from the boundary layer flow over the surface of the aircraft detaching from the surface as the flow leaves the aircraft (i.e. mixing losses).
Reducing the size of, or eliminating the body wake altogether reduces the losses generated in the wake and also in the main jet by reducing the thrust requirements from the main engines.
The concept of Fuselage Boundary Layer Ingestion places the aircraft's engine at or immediately behind the tail of the aircraft so as to ingest the boundary layer flow as it leaves the aircraft's fuselage surface. In this way, the downstream mixing losses in the fuselage wake can be eliminated or significantly reduced.
However, this approach requires the heavy and bulky engines to be mounted at the rear of the aircraft, which makes the construction and operation of the aircraft less practical and less cost effective for a user.
Even the use of a small boundary layer ingesting fan placed at the aft fuselage can provide significant fuel savings in operation of the aircraft. A gas turbine engine is impractical for such supplementary propulsor operation because of the associated weight and space requirements. An electrical propulsion source is preferred due to its high transmission efficiency and reliability. However, a conventional electrical machine in a direct drive arrangement would be heavy, and the additional of a gearbox would further increase complexity and require additional space.